solves binary tree level order traversal in java

Author: anishLearnsToCode

README.md

@@ -93,7 +93,7 @@
 |  99  | [Recover Binary Search Tree](https://leetcode.com/problems/recover-binary-search-tree)                                                                    |                                                                                                                                                                                      |                                                                                                                                   |
 | 100  | [Same Tree](https://leetcode.com/problems/same-tree)                                                                                                      |                                          [![Java](assets/java.png)](src/SameTree.java) [![Python](assets/python.png)](python/same_tree.py)                                           |                                                                                                                                   |
 | 101  | [Symmetric Tree](https://leetcode.com/problems/symmetric-tree)                                                                                            |                                     [![Java](assets/java.png)](src/SymmetricTree.java) [![Python](assets/python.png)](python/symmetric_tree.py)                                      |                                                                                                                                   |
-| 102  | [Binary Tree Level Order Traversal](https://leetcode.com/problems/binary-tree-level-order-traversal)                                                      |                                                                                                                                                                                      |                                                                                                                                   |
+| 102  | [Binary Tree Level Order Traversal](https://leetcode.com/problems/binary-tree-level-order-traversal)                                                      |                                                          [![Java](assets/java.png)](src/BinaryTreeLevelOrderTraversal.java)                                                          |                                                                                                                                   |
 | 103  | [Binary Tree Zigzag Level Order Traversal](https://leetcode.com/problems/binary-tree-zigzag-level-order-traversal)                                        |                                                                                                                                                                                      |                                                                                                                                   |
 | 104  | [Maximum Depth of Binary Tree](https://leetcode.com/problems/maximum-depth-of-binary-tree)                                                                |                         [![Java](assets/java.png)](src/MaximumDepthOfBinaryTree.java) [![Python](assets/python.png)](python/maximum_depth_of_binary_tree.py)                         |                                                                                                                                   |
 | 105  | [Construct Binary Tree from Preorder and Inorder Traversal](https://leetcode.com/problems/construct-binary-tree-from-preorder-and-inorder-traversal)      |                                                                                                                                                                                      |                                                                                                                                   |

src/BinaryTreeLevelOrderTraversal.java

@@ -0,0 +1,38 @@
+// https://leetcode.com/problems/binary-tree-level-order-traversal
+// T: O(n)
+// S: O(n)
+
+import java.util.ArrayList;
+import java.util.LinkedList;
+import java.util.List;
+import java.util.Queue;
+
+public class BinaryTreeLevelOrderTraversal {
+    public List<List<Integer>> levelOrder(TreeNode root) {
+        if (root == null) {
+            return List.of();
+        }
+
+        final Queue<TreeNode> queue = new LinkedList<>();
+        queue.add(root);
+        queue.add(null);
+        final List<List<Integer>> result = new ArrayList<>();
+        List<Integer> level = new ArrayList<>();
+
+        while (!queue.isEmpty()) {
+            TreeNode current = queue.poll();
+            if (current == null) {
+                result.add(level);
+                if (!queue.isEmpty())  {
+                    queue.add(null);
+                    level = new ArrayList<>();
+                }
+            } else {
+                level.add(current.val);
+                if (current.left != null) queue.add(current.left);
+                if (current.right != null) queue.add(current.right);
+            }
+        }
+        return result;
+    }
+}